"Scanning with compensation" is a method of radar tracking which, like the more familiar "conical scanning tracking", generates a tracking error signal from the modulation of a target echo which results from rotating about the antenna scanning axis an antenna pattern which is offset in angle or "squinted" by an amount which is typically one half to one quarter of the half-power one way beam width of the antenna. Unlike conical-scan tracking, the antenna feed is used to generate two pencil-beam radiation patterns which are squinted by equal but opposite angles with respect to the scan axis. This type of radiation pattern is illustrated in FIG. 1. During transmission these two pencil-beam patterns are summed coherently to give a resultant approximately steady illumination field, i.e., for a considerable angular region centered on the scan axis, the field intensity will be nearly constant and unmodulated by the scanning rotation of the feed. During reception, however, the two pencil-beams are separately sensed, and each is scan-modulated in amplitude by an amount which depends on the angular distance of the target from the scan axis and with a phase which depends on the rotation of the feed around the scan axis. Because of the equal and opposite squint angles of the two beams, their modulation phases will necessarily be 180.degree. out of phase with each other. The modulation on the two receiving beams is then processed to derive tracking error signals in two orthogonal planes in a manner generally similar to that used in conical scan tracking. However, it is recognized that tracking using the method of "scanning with compensation" is free from errors caused by target echo amplitude fluctuations (scintillation) which are present in conical-scan tracking and further that "scanning with compensation" tracking is highly resistant to a number of ECM techniques such as "inverse-gain modulation" which disrupts conical scan tracking.
A known method of producing the equal but opposite squinted beams for the method of "scanning with compensation" is to use two rectangular waveguides with a common wall (usually the broad wall, which results in a more desirable cross-over level than would the narrow wall). Alternatively, rectangular horns fed by rectangular waveguides may be used as the feed radiating elements, similarly joined along one edge of their apertures. This feed assembly rotates for scanning the two beams, about an axis parallel to the direction of radiation and passing through the midpoint of their common wall. The two waveguides are connected to the non-rotating part of the antenna feed through a dual-channel rotary joint of any of a number of known types. The non-rotating inputs to the rotary joint are connected by microwave switches, hybrid bridges, and/or circulators so that the two apertures are connected in phase during transmission but disconnected from each other and separately connected to the receiver during reception. Because the rectangular waveguides have only a single fixed polarization, the plane of polarization of this feed will rotate at the same rate as the squinted beams rotate about the scan axis.